Re: DPI
- From: "DBLEXPOSURE" <celstuff@xxxxxxxxxxx>
- Date: Tue, 1 May 2007 21:03:45 -0400
Patrick Ziegler ImageQuest Photography
"Nospam" <nospam@xxxxxxxxxxxxxxxxxx> wrote in message
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In message <I6GdnWlniIkP1avbnZ2dnUVZ_sSmnZ2d@xxxxxxxxxxxx>, DBLEXPOSURE
<celstuff@xxxxxxxxxxx> writes
Patrick Ziegler ImageQuest Photography
"Nospam" <nospam@xxxxxxxxxxxxxxxxxx> wrote in message
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Could some one tell me in simple language or point me to a web site to
answer the following question.
I have read that cameras work on 72 dpi
so how do u print to 300 dpi?
Not sure about the technical side yet.
Maybe I'm mixing up the wrong things!
thanks
--
Nospam
OMG not another DPI thread...
Just kidding, here is the answer I posted in a thread above.
It is really simple so all you folks who are scratching your head, pay
attention.
The pixel originates in your camera. If you have an 8MP camera then guess
what? There are roughly eight million Pixels on the sensor in your
camera.
Those pixels are laid out in a matrix, so many wide by so many tall.
I'll use the ever-popular EOS 400D as an example. It sensor is 3888 x
2592
pixels.
Guess what? 3888 x 2592 = 10,077,696 or 10.1 million.
SO now we have this image we have captured with our 10MP camera and it is
3888 x 2593. It's a big file.
A typical 20inch monitor might have 1600 x 1200pixels, (Native
resolution).
Now wait, how am I going to fit 3888 pixels across a screen that is only
1600 pixels wide?
When display that large file on that relatively small screen, whatever
software you are using to display the image will re-sample it and squish
it
down to a manageable size to display.
In Photoshop and other programs, this can lead to an image that has a bit
of
aliasing and other artifacts do to an algorithm that is designed to be
fast.
But no worries, the algorithms has not changed the file, it has only
manipulated the file existing in RAM.
Okay, so we still have our large image and we want to keep that image if
we
intend on printing it. Why? You ask.
Because the more pixels we have the larger we can print and retain all the
quality and detail that was captured by the camera.
300PPI, sometimes written 300DPI, Yes, people do use the wrong terminology
from time to time but lets face it, a pixel is a dot, is the highest
resolution discernable by the human eye. 300 is used as a reference point
when we talk about making prints of our images.
If we print our 3888 pixel wide image at 300ppi it will be nearly 13inches
wide. 3888pixels/300pixels per inch = 12.96
But wait, I need it to be 15inches wide. No problem, it will then print
at
259ppi
3888 / 15 = 259
3rd grade math.
How low can we go? It is up to you and the size of your printer. If you
have a photo of a black square on a white wall, you can print it at 80ppi
and it will still have sharp edges and nice corners.
Now, if you shoot a wedding and print the brides portrait at 80ppi your
probably going to end up in court. Detail and the viewer's eye for
quality
determine the threshold. But remember, nobody can see better than 300ppi.
Okay, but I am not going to print this photo I only want to use it on my
Web
site. Re-size it in Photoshop. Image>image size>resample your screen
resolution and the size you want it to appear on that screen will
determine
what pixel dimensions you choose. I typically make mine either 800wide or
600tall depending on if they are horizontal or vertical compositions.
But wait, my new printer says in has a Max Resolution of 4800 X 2400DPI
you
said nobody can see better than 300, what is up with that?
Totally different spec, somewhat related but different.
Long story short, This is what I tell people so I don't have to get that
deer in the headlights look, Printer resolution is how many dots of ink
the
printer uses to your image's pixels. So, if we print at 300ppi on this
new
printer at 4800dpi we will get 16 dots of ink in ever pixel that was
captured by the camera. This means better tonal quality not necessarily
sharper image.
4800 / 300 = 16 more 3rd grade math.
That's it, long winded but not overly complicated.
PZ
www.imagequest.ifp3.com
Thanks for that info, easy to follow, explained well - if only I could
get the process of photosynthesis over to my year 9 student, very
helpful info.
Question
So if my printer is printing 16 dots of ink per pixel.
Well this seams a lot of dots of ink to a pixel, the dots of ink must be
very small.
What number of dots per inch can the eye resolve? I'm not sure if this
is actually a sensible question as I think it is linked to the original
resolution.
I have to admit to being a digital freak when it computers, DVD etc.,
love the stuff.
But when it comes to photography I am still trying to hang onto my
'analogue' equipment, I love my T90 and my darkroom with all its
chemicals etc..
But...
while on holiday I did, test the digital waters so to speak, and bought
an Olympus mju 7600.
Just a small compact happy snappy unit, able to take with me anywhere I
go. Produces some nice pics but I have a lot to learn about it yet, only
had it 3 weeks.
I know my T90 and A1 inside out.
So a lot to learn about the 7600 and digital photography yet.
But I can see lots of advantages with this new fangle technology.
I may move to a DSLR later. But for now still lots to get my head
around.
Now just for fun,, and to clarify my understanding a bit...
The spec of my camera says it has a ccd sensor size of
1/2.33" CCD (primary colour filter), 7,380,000 (gross)
I assume that means the diagonal is 1/2.33" = 0.429"
so this gives an aspect ratio of 3072 × 2304 (1.3333 : 1)
so I think this must mean that the CCD sensor is
about 8.72 mm X 6.54 mm approx. (25.4 mm to the inch)
the camera spec say;
SHQ 3072 × 2304 Low compression
HQ 3072 × 2304 Standard compression
(and some other I have not bothered with why have
7MP and not use them xD cards are cheap now.)
I have been using the SHQ mode only.
so that mean I have 3072 pixels over 8.72 mm so that means
I have a CCD sensor resolution of
3072/8.72 * 25.4 = 8948 ppi
if I'm right, but it does sound a bit high!
I realise this is sensor resolution and not image or printing
resolution.
BTW I have had a look at your web site, nice photos, the quality is
nearly as good as 'real' photography ;-) I love my T90, all that money
now obsolete, boo hoo.
--
Nospam
Thank you for the compliment, some of the photos on my site where captured
with color-slide film.
It is a good question; 300ppi is the max resolution the best eye can
resolve.
Your math and reasoning seams accurate with looking further and picking it
apart, your conclusions are correct.
The ppi of your camera's sensor is important to the performance of your
camera. The race for engineers to increase pixel density in camera sensors
has been going on for a while.
However, I have not heard or read the topic being discussed using that
terminolgy, not to say it doesn't happen, I just haven't heard it put that
way.
In the DSLR world, we talk about Mega Pixels. Most DSLR's have a sensor of
similar physical size. From Canon's Rebel XT, "This APS-C size sensor (22.2
x 14.8mm) has the same 3:2 ratio as film cameras, creating an effective
angle of view that is 1.6x the normal EF Lens focal length"
What this means is the XT's sensor has 8MP and is smaller than a 35mm frame.
The Rebel Xti, "22.2 x 14.8mm (APS-C size), 10.5 Megapixel" So, this camera
must have a higher ppi spec. Manufactures do not publish the spec they just
give you the MP and the sensor size. I suppose somewhere along the way they
do talk about pixel density.
The Canon EOS 1DS Mark II, "36.0 x 24.0mm 17.2 megapixel" even higher yet
and full frame sensor. Full frame meaning the sensor is the same size as a
35mm frame (or very close). This means when you use a 100mm lens on this
camera it acts like a 100mm lens, not 160mm as the lens would on an APS-C
sensor.
To keep the waters from getting muddy, don't think about the sensor the
image and the printer at once and try to keep the resolution numbers making
sense.
You don't look at a photo on the camera sensor so the 300ppi benchmark is
meaningless when you think about sensors.
300ppi is what you think about when you go to print an image of a given
pixel dimension and make a decision about how large you can print it and
maintain quality. Some images can be printed at 100 ppi and look great,
others with significant detail will have to be printed closer to that 300
benchmark if you want to retain the detail.
When you go to buy a printer, the higher DPI rating will help you know what
kind of tonal and shading quality you can expect. (The more dots a printer
can fit in one inch, the more accurately the printer can reproduce tones and
shading) That is not to say that this spec is the only printer spec you
should pay attention to.
I am a photographer and not an engineer for Epson or Canon so my explanation
may not be 100% accurate and I am sure others will chime in with detailed
corrections or obtuse explanations but I think I have got the concept down
pretty well.
The best way to get a grip on this stuff is to get your hands on it. Shoot
with a 6MP and 8 and 10 and you will get it when you start working the
images in Photoshop and printing them. Print with a $40 Lexmark and then a
higher end photo printer, you will start to understand.
Patrick Ziegler
www.imagequest.ifp3.com
.
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